Low-Level Jet Development during a Numerically Simulated Return Flow Event

Abstract

The evolution of the southerly low-level jet (LLJ) during a return flow event is studied using output from the Penn State/NCAR Mesoscale Model (Version 4). Three geographically different southerly LLJs develop in the simulation: one over the southern Plains of the United States, a second southwest of Brownsville, Texas, and a third over the western Gulf of Mexico. The LLJ over the Plains is found to form first as an inertial oscillation and later as a response to lee troughing and an elevated mixed layer that develops over the region. Over Mexico, the temperature structure over the Altiplanicie Mexicana (Mexican High Plain) is responsible for a locally intense low-level pressure gradient east of the High Plain which remains nearly stationary over two diurnal cycles. The LLJ over the western Gulf of Mexico results largely from topographic blocking of the low-level southerly flow by the eastern end of the Neovolcanic Cordillera northwest of Veracruz, Mexico.The evolution of the lower troposphere over the southern Plains resembles the Carlson and Ludlam conceptual model for a severe storm environment, but the structure of the return flow is complex. When midlevel westerlies are weak, mesoscale and boundary layer processes govern the development of LLJs. As the west and southwesterly winds increase with an approaching upper-level disturbance, synoptic influences overwhelm the mesoscale processes leading to a single, larger-scale LLJ.

An NARR-Derived Climatology of Southerly and Northerly Low-Level Jets over North America and Coastal Environs

“…However, few of these studies included vertical shear criteria as part of the jet definition, rather identifying jets from wind FIG. Support for this interpretation includes the 3-day numerical simulations by Igau and Nielsen-Gammon (1998) of S-LLJs that formed over Brownsville, Texas, and near Veracruz, Mexico, in early April during a ''return flow event'' with warm, moist air drawn northward ahead of a developing upper-level trough. As in Fig.…”

Characteristics of Low-Level Jets over Northern Taiwan in Mei-Yu Season and Their Relationship to Heavy Rain Events

“…Several mechanisms have been put forward from past studies as the responsible process for the development and intensification of LLJs under different conditions. Over the United States, LLJs often develop in response to an enhanced pressure gradient at low levels, under a thermally indirect transverse circulation associated with an upper-level jet (ULJ) streak at its exit region (Uccellini and Johnson 1979;Uccellini 1980;T.-C. Chen and Kpaeyeh 1993), or to the southeast of the center of a deepening extratropical cyclone (or trough) at the lee of the Rockies through thermal wind adjustment (Arritt et al 1997;Igau and Nielsen-Gammon 1998). During these processes, diabatic effects from latent heat release (and surface fluxes) can also join the interaction and contribute positively toward the LLJ formation (e.g., Gall 1976;Emanuel et al 1987;Uccellini et al 1987).…”

“…In addition to the aforementioned mechanisms, orographic blocking on prevailing air flow at a low Froude number (Fr) flow regime can often produce local wind maxima inside the PBL at various regions around the world (e.g., Smith and Grubišić 1993;Georgelin et al 1996;Douglas et al 1998;Igau and Nielsen-Gammon 1998;Li and Chen 1998;Skamarock et al 1999;Chelton et al 2000). These wind maxima forced by terrain, however, often occur within 1 km above the surface and represent a different class of jets, and perhaps should be referred to as ''barrier jets'' (BJs).…”

Relationships of coastal nocturnal boundary layer winds and turbulence to Houston ozone concentrations during TexAQS 2006

“…Under both kinds of flow conditions, the afternoon sea breeze, or bay breeze, which occurs almost daily in Houston, typically progresses into a nocturnal low‐level jet in what Nielsen‐Gammon [2006] refers to as the “sea breeze low‐level jet.” This jet is similar to the summertime Great Plains LLJ [ Parish et al , 1988; Holton , 1967; McNider and Pielke , 1981] in the sense that both are a result of topographically driven baroclinity. However, the close relationship with the sea breeze makes the LLJs in Houston physically distinct from the Great Plains LLJs or the synoptically forced LLJs over Texas and the western Gulf of Mexico [ Uccellini and Johnson , 1979; Djurić and Damiani , 1980; Igau and Nielsen‐Gammon , 1998]. Some of the weaker Houston sea breeze LLJs observed during TexAQS 2006 occurred only locally, independent of these other LLJs, such that they were not observable with profilers located further inland or further out to sea.…”